one. Foreword:
Servo system is an important part of electromechanical products, it can provide the highest level of dynamic response and torque density, so the development trend of the drive system is to use AC servo drive to replace the traditional hydraulic, DC, stepper and AC variable speed drive In order to achieve a new level of system performance, including shorter cycles, higher productivity, better reliability and longer life. In order to realize the better performance of the servo motor, it is necessary to understand some characteristics of the use of the servo motor. This article will analyze the common problems of servo motor in use.
two. Problem 1: Noise, Unstable When customers use servomotors on some machines, noise often occurs too much and the motor drives the load to run unstable. When this problem occurs, the first reaction of many users is the quality of the servo motor. It's not good, because sometimes a stepping motor or a variable frequency motor is used to drag the load, but the noise and instability are much smaller. On the surface, it is indeed the cause of the servo motor, but after careful analysis of the working principle of the servo motor, we will find that this conclusion is completely wrong.
AC servo system includes: servo drive, servo motor and a feedback sensor (general servo motor comes with optical coder). All of these components operate in a controlled closed loop system: the drive receives parameter information from the outside and then delivers a constant current to the motor. The motor converts the torque into a load. The load acts or accelerates and decelerates according to its own characteristics. The sensor measures the load. The position of the drive device compares the setting information value with the actual position value, and then changes the motor current to make the actual position value and the setting information value consistent. When the load suddenly changes and the speed changes, the coder knows this. After the speed changes, it will immediately respond to the servo driver. The driver will also change the load value by changing the current value provided to the servo motor and return to the set speed again. The AC servo system is a highly-responsive full-closed-loop system. The time lag between the load fluctuation and the correct speed is very fast. At this time, the time that actually limits the response of the system is the transit time of the mechanical connection device.
As a simple example, there is a machine that uses a servo motor to drive a constant speed, high inertia load through a V-belt. The entire system needs to obtain constant speed and faster response characteristics and analyze its action process:
When the driver sends current to the motor, the motor immediately generates torque; at the beginning, because the V-belt will have elasticity, the load will not accelerate as fast as the motor; the servo motor will reach the set speed earlier than the load, this fashion is The encoder on the motor will weaken the current and then weaken the torque. As the V-belt tension increases, the speed of the motor will slow down. At this time, the drive will increase the current and start again.
In this example, the system is oscillating, motor torque is fluctuating, and load speed fluctuates. The result will certainly be noise, wear, and instability. However, this is not caused by a servo motor. This noise and instability is due to the mechanical transmission. It is caused by the fact that the response speed (high) of the servo system does not match the mechanical transmission or the reaction time (longer). That is, the servo motor response is faster than the time required for the system to adjust the new torque.
Finding the root cause of the problem is of course much easier to solve. For the above example, you can: (1) increase the rigidity of the machine and reduce the inertia of the system, and reduce the response time of the mechanical transmission parts, such as replacing V-belts with direct wires. Rod drive or gear box instead of V-belt. (2) Reduce the response speed of the servo system and reduce the control bandwidth of the servo system, such as reducing the gain parameter value of the servo system.
Of course, the above is just one of the causes of noise and instability. For different reasons, there are different solutions, such as noise caused by mechanical resonance, resonance suppression in the servo, low-pass filtering, etc. The causes of noise and instability are basically not due to the servo motor itself.
three. Problem 2: Inertia Matching In the servo system selection and debugging, the inertia problem is often encountered! Specifically: 1 In the servo system selection, in addition to taking into account the motor torque and rated speed and other factors, we also need to first calculate the mechanical system converted to the motor shaft inertia, and then according to the actual action of the machine requirements and processing The quality requirements for the specific choice of motor with a suitable inertia size; 2 In the debugging (manual mode), the correct setting of inertia ratio parameters is to give full play to the mechanical and servo system optimal performance of the premise, this point in the requirements of high speed and high precision The performance on the system is highlighted by (the Delta servo inertia ratio is 1-37, JL/JM). In this way, there is a problem of inertia matching!
What exactly is "inertia matching"?
1. According to Newton's second law: “The torque required for the feed system T = The system transmission inertia J × The angular acceleration θ The angular acceleration θ influences the dynamic characteristics of the system. The smaller θ, the time from when the controller issues an instruction to when the system finishes executing. The longer the system reacts, the slower it will be, and if θ changes, the system response will be fast and slow, affecting the machining accuracy.Because the maximum output T value is constant after the motor is selected, if the change of θ is small, J should be as small as possible. 2. Total Inertia of Feed Axis "J = Rotational Inertial Momentum of Servomotor JM + Load Inertia Momentum of Motor Shaft Conversion JL Load Inertia JL (For example, power tool) Workbench, jig and workpiece, screw, The inertia of the linear and rotary moving parts such as the coupling is converted to the inertia of the motor shaft. JM is the rotor inertia of servo motor. After the servo motor is selected, this value is the fixed value, and JL changes with the load of the workpiece and so on. If you want J to have a smaller rate of change, it is best to make JL smaller. This is the "inertia match" in the popular sense.
Knowing what is the inertia matching, how does the inertia matching have any effect?
1. Influence: The transmission inertia has influence on the precision, stability and dynamic response of the servo system. Large inertia, large mechanical constants of the system, and slow response will reduce the natural frequency of the system and cause resonance, thus limiting the servo bandwidth. With servo accuracy and response speed, an appropriate increase in inertia is only beneficial when improving low-speed creep. Therefore, mechanical design should minimize the inertia without affecting system stiffness.
2. Determine: Measure the dynamic characteristics of the mechanical system, the smaller the inertia, the better the dynamic response of the system; the greater the inertia, the greater the load of the motor, the more difficult to control, but the inertia of the mechanical system must match the motor inertia Row. Different institutions have different choices for the principle of inertia matching and have different performances. For example, when a CNC center machine uses a servo motor for high-speed cutting, when the load inertia increases, it will happen: 1. When the control command is changed, the motor needs more time to reach the speed requirement of the new command; 2. When the machine is along the second When the shaft performs arc cutting, a large error occurs. 1. General Servo Motors In the normal condition, when JL ≦ JM, the above problem does not occur. 2. When JL = 3×JM, the controllability of the motor will be slightly reduced, but it will have no effect on ordinary metal cutting. (High speed curve cutting general recommendation JL ≦ JM)
3. When the JL ≧ 3 × JM, the controllability of the motor will be significantly reduced, the performance of the different institutions in the high-speed curve cutting and the processing quality requirements have different requirements on the JL and JM size relationship, the determination of inertial matching needs to be based on Mechanical process characteristics and processing quality requirements to determine.
four. Problem 3: Servo Motor Selection After selecting a mechanical transmission scheme, it is necessary to select and confirm the servo motor model and size. (1) Selection conditions: Under normal circumstances, the following conditions should be satisfied when selecting the servo motor:
1. The maximum speed of the motor > the highest moving speed required by the system.
2. The rotor inertia of the motor matches the load inertia.
3 continuous load working torque torque rated torque
4. Maximum output torque of the motor> Maximum torque required by the system (torque at acceleration)
(2) Calculation of type selection:
1. Inertia Match Calculation (JL/JM)
2. Calculation of rotation speed (load end speed, motor end speed)
Load torque calculation (continuous load working torque, torque at acceleration)